Accuracy of implantation of components in the Oxford knee using the minimally invasive approach.

Screened postoperative X-rays of 224 Oxford knees implanted through the minimally invasive approach were analysed using 16 criteria. The technique was as recommended by the Oxford Group except that the femoral intramedullary rod was used only as a guide to flexion/extension of the femoral component. All femoral components were within the recommended range for varus/valgus and mediolateral position. Eighteen femoral components were either too flexed or extended, but by a maximum of only 10 degrees . Tibial components were inserted in slight varus (mean 1.8 degrees , S.D. 8.8, range 10 to -10). All were within the range for posterior slope in spite of difficulty in establishing objective landmarks. Tibial coverage was imprecise with the phase 3 implants which were short and broad relative to the cut surface. In 36% of knees the tibial tray was implanted too anterior resulting in posterior under hang. There have been no clinical signs to indicate tipping of the meniscus in deep flexion. The depth of cement penetration under the keel was excessive in three cases, resulting from porotic bone rather than technical error in the depth of the cut. There has been one case of femoral component loosening following a fall and no tibial loosening in spite of imprecise alignment of some components.